As is well known and understood, perhaps the most common method of reducing continuous wave (CW) interference is through the use of bandpass notch filters. However, as applied in audio communications systems and particularly in defense communication systems, the use of such filters have certain inherent limitations. For example, relatively large and, therefore, expensive inductors and capacitors are required for passive, low frequency filtering. Additionally, the filtering also tends to distort the voice signal and thus deteriorates its intelligibility. Furthermore, the filtering is done around a preset frequency which permits the signal to be jammed merely by changing frequencies.
One known type of interference cancelling system is shown and described in U.S. Pat. No. 4,027,264, entitled, "Phase Lock Loop Multi-tone Interference Cancelling System", issued to Frank S. Gutleber, the present inventor, on May 31, 1977. The interference cancelling system of that invention employs a phase lock loop which is utilized as a narrow band tracking filter to adaptively lock to interference tones to be cancelled by a process of signal subtraction.
As noted above, the present invention relates to communication systems utilizing noise coded signals. Such systems, furthermore, are well known and are particularly desirable because they exhibit an immunity against self-interference and jamming. Noise coded means that the information is coded with a code that is "noise like" in that it will compress to an impulse when detected with a matched filter. In particular, one class of noise codes are known wherein pairs of coded signals termed "code mates" have autocorrelation functions which provide a peak output at a given time and a zero output or outputs having the same magnitude but opposite polarity at all other times. When the code mates, for example, are multiplexed, matched filter detected and linearly added, there is provided a lobeless impulse output of a relatively high amplitude at a given time and a zero output at all other times. Such codes and systems utilizing such codes are typically shown and described in U.S. Pat. No. 3,461,451, entitled, "Code Generator To Produce Permutations of Code Mates", which issued to F. S. Gutleber on Aug. 12, 1969; U.S. Pat. No. 3,634,765, entitled, "Systems to Provide An Impulse Autocorrelation Function . . . Of One Or More Of Said Code Signals", which issued to F. S. Gutleber on Jan. 11, 1972; and U.S. Pat. No. 3,908,088, entitled, "Time Division Multiple Access Communication System", which issued to F. S. Gutleber on Sept. 23, 1975.